Siemens siprotec 7SD5 User Manual page 101

Pickup Value When switching on long, unloaded cables, overhead lines and arc-compensated lines,
during Switch-on considerable higher-frequency transient reactions may occur. These peaks are con-
siderably damped by means of a digital filter in the differential protection. A pickup
value I-DIF>SWITCH ON (address 1213) can be sent to reliably prevent single-sided
pickup of the protection. This pickup value is always active when a device has recog-
nized the connection of a dead line at its end. For the duration of the seal-in time SI
Time all Cl. which was set with the general protection data under address 1132
(Section 2.1.4.1 all devices are then switched over to this particular pickup value. A
setting to three to four times the steady-state charging current ensures usually the sta-
bility of the protection during switch-on of the line. For switch-on of a transformer or
shunt reactor, an inrush restraint is incorporated (see below under margin heading
„Inrush Restraint").
Final checks will be carried out during commissioning. Further information can be
found in chapter Installation and Commissioning.
Time Delays In special cases of application it may be useful to delay the tripping of the differential
protection with an additional timer, e.g. in case of reverse interlocking. The delay time
T-DELAY I-DIFF> (address 1217) is only started upon detection of an internal fault.
This parameter can only be altered with DIGSI
If the differential protection is applied to an isolated or resonant-earthed network, it
must be ensured that tripping is avoided during the transient oscillations of a single
earth fault. With address 1218 T3I0 1PHAS the pickup to a single fault is therefore
delayed for 0.04 s (presetting). For large resonant-earthed systems the time delay
should be increased. By setting the address to ∞ the single-phase tripping is totally
suppressed.
Please note that the parameter T3I0 1PHAS is also used by the distance protection
function. The settings that you make here also affect the distance protection (see
Section 2.5.1.4 under margin heading „Earth Fault Detection").
The pickup threshold of the charge comparison stage is set in address 1233 I-
Pickup Value of
DIFF>>. The RMS value of the current is decisive. The conversion into charge value
Charge Compari-
son Stage is carried out by the device itself.
Setting near the operational nominal current is adequate in most cases. Please also
remember that the setting is related to the operational nominal values that must be
equal (primary) at all ends of the protected object.
Since this stage reacts very fast, a pickup of capacitive charging currents (for lines)
and inductive magnetizing currents (for transformers or reactors) — also for switch-on
condition — must be excluded. This is also true when the charging current compen-
sation is on, since the compensation is not effective for charge comparison.
In resonant-earthed systems the value of the non-compensated system earth fault
current should also not be undershot. This value derives from the total capacitive earth
fault current without considering the Petersen coil. As the Petersen coil serves to com-
pensate nearly the total earth fault current, its nominal current can be taken as a base.
For transformers set the value I
The pickup thresholds are finally checked during commissioning. For further informa-
tion please refer to the chapter Installation and Commissioning.
Charging Current The charging current compensation can only operate if this function has been set
during configuration of the functional scope (Section 2.1.2) at address 149 charge I
Compensation
comp. = Enabled. Also, the line data must have been configured (Section 2.1.4.1).
7SD5 Manual
C53000-G1176-C169-1
®
under Additional Settings.
/U .
N Transf k Transf
2.3 Differential Protection
101